1. Field of the Invention
The present invention relates to a motor drive apparatus for controlling a motor in which power to drive the motor is provided by first converting AC power supplied from an AC power supply to DC power and then converting the DC power back to AC power, and more specifically to a motor drive apparatus equipped with an energy storage unit that can store the power supplied from the AC power supply as well as the power regenerated by the motor and can supply the stored power to the motor.
2. Description of the Related Art
In a machine tool system, motors are provided one for each drive shaft of a machine tool, and the motors are driven and controlled by a motor drive apparatus. The motor drive apparatus commands and controls the motor speed, torque, or rotor position of each of the motors that drive the respective drive shafts of the machine tool.
The motor drive apparatus includes an AC/DC converter which converts three-phase AC utility power to DC power, a DC/AC converter which converts DC power to AC power of a desired frequency for driving a motor or converts AC power regenerated by the motor back to DC power, and a DC link unit which connects the DC side of the AC/DC converter to the DC side of the DC/AC converter and delivers the DC power from one to the other and vice versa, and the motor drive apparatus controls the motor speed, torque, or rotor position of the motor connected to the AC side of the DC/AC converter.
In many cases, as many DC/AC converters are provided as there are the motors so that the motors connected to the respective drive shafts of the machine tool can be driven and controlled independently of each other, but in some cases, more than one is provided for each motor. On the other hand, as for the AC/DC converter, usually only one AC/DC converter is provided for the plurality of DC/AC converters in order to save the cost and installation space of the motor drive apparatus.
When controlling motor acceleration or deceleration by using such a motor drive apparatus, a power peak occurs because a large AC power output or regeneration is required of the AC power supply. To suppress such power peaks, an energy storage device that can store regenerative power from the motor during deceleration and can reuse the stored power during motor acceleration may be provided on the DC link. In this case, since the AC power regenerated by the motor needs to be converted to DC power that can be stored in the energy storage device, the DC/AC converter is constructed from a semiconductor power converter that can perform conversion in both directions, i.e., not only from DC to AC but also from AC to DC.
As an example, an energy storage device using a flywheel is disclosed in Japanese Patent Unexamined Publication No. 2008-023599. FIG. 6 is a diagram showing one example of a motor drive apparatus using a flywheel according to the prior art. It is to be understood that, throughout the different drawings given herein, the same reference numerals designate component elements having the same functions. The motor drive apparatus 100 using a flywheel according to the prior art includes an AC/DC converter 11 which converts AC power supplied from an AC power supply 3 to DC power, a DC/AC converter 12 which converts DC power to AC power for driving a motor 2, a DC link unit 13 which connects the DC side of the AC/DC converter 11 to the DC side of the DC/AC converter 12 and delivers the DC power from one to the other and vice versa, a flywheel storage unit 51 which is connected to the DC link unit 13 and which stores or outputs the DC power, and a control unit 52 which controls the speed, torque, or rotor position of the motor 2 in accordance with a motor operation command for commanding the operation of the motor 2, and which also controls the amount of DC power to be stored or output by the flywheel storage unit 51. Actually, there are as many DC/AC converters 12 as there are the motors 2. For example, the drive shafts of a machine tool are connected one for one to the motors 2. Each DC/AC converter 12 whose switching operation is controlled by the motor operation command from the control unit 52 outputs an alternating current having a frequency and waveform for achieving desired motor operation. The motor 2 is supplied with the alternating current output from the DC/AC converter 12, and the motor 2 is thus driven for rotation. The flywheel storage unit 51 includes an inertia rotating motor 51-1, an inertia 51-2, and a DC/AC converter 51-3. The control unit 52 supplies the DC/AC converter 51-3 with a command for controlling the amount of DC power to be stored or output by the flywheel storage unit 51. If the received command is one that commands the flywheel storage unit 51 to output the DC power, the DC/AC converter 51-3 performs forward conversion from AC to DC so that the energy stored in the inertia 51-2 flows to the DC link unit 13; on the other hand, if the received command is one that commands the flywheel storage unit 51 to store the DC power, the DC/AC converter 51-3 performs reverse conversion from DC to AC so that the energy from the DC link unit 13 flows to the inertia rotating motor 51-1, thereby causing the inertia rotating motor 51-1 mounted with the inertia 51-2 to rotate. In this way, the supplied power energy is stored as the rotational energy of the inertia 51-2. With this arrangement, during acceleration of the motor 2, not only the energy from the AC power supply 3 but also the energy stored in the inertia storage unit 51 is supplied to the motor 2 and used as the power to accelerate the motor 2, and during deceleration of the motor 2, the regenerative energy from the motor 2 flows into the flywheel storage unit 51 for storing therein. The energy stored in the flywheel storage unit 51 is reused when accelerating the motor 2.
As another example, an energy storage device using a capacitor is disclosed in Japanese Patent No. 4339916. FIG. 7 is a diagram showing one example of an energy storage device using a capacitor according to the prior art. The motor drive apparatus 100 using a capacitor according to the prior art includes an AC/DC converter 11 which converts AC power supplied from an AC power supply 3 to DC power, a DC/AC converter 12 which converts the DC power to AC power for driving a motor 2, a DC link unit 13 which connects the DC side of the AC/DC converter 11 to the DC side of the DC/AC convertor 12 and delivers the DC power from one to the other and vice versa, a capacitor storage unit 61 which is connected to the DC link unit 13 and which stores or outputs the DC power, an energy control unit 62 which controls the amount of DC power to be stored or output by the capacitor storage unit 61, and a motor control unit 63 which controls the speed, torque, or rotor position of the motor 2 in accordance with a motor operation command for commanding the operation of the motor 2. Actually, there are as many DC/AC converters 12 as there are the motors 2. For example, the drive shafts of a machine tool are connected one for one to the motors 2. Each DC/AC converter 12 whose switching operation is controlled by the motor operation command from the motor control unit 63 outputs an alternating current having a frequency and waveform for achieving desired motor operation. The motor 2 is supplied with the alternating current output from the DC/AC converter 12, and the motor 2 is thus driven for rotation. The capacitor storage unit 51 includes a large-capacitance capacitor 61-1 and a DC-DC converter 61-2. The energy control unit 62 supplies the DC-DC converter 61-2 with a command for controlling the amount of DC power to be stored or output by the capacitor storage unit 61. If the received command is one that commands the capacitor storage unit 61 to output the DC power, the DC-DC converter 61-2 operates so as to make the DC voltage at the side connected to the capacitor 61-1 higher than the DC, voltage at the side connected to the DC link unit 13 so that the energy stored in the capacitor 61-1 flows to the DC link unit 13. On the other hand, if the received command is one that commands the capacitor storage unit 61 to store the DC power, the DC-DC converter 61-2 operates so as to make the DC voltage at the side connected to the capacitor 61-1 lower than the DC voltage at the side connected to the DC link unit 13 so that the energy from the DC link unit 13 flows to the capacitor 61-1 and the energy is thus stored in the capacitor 61-1. With this arrangement, during acceleration of the motor 2, not only the energy from the AC power supply 3 but also the energy stored in the capacitor storage unit 61 is supplied to the motor 2 and used as the power to accelerate the motor 2, and during deceleration of the motor 2, the regenerative energy from the motor 2 flows into the capacitor storage unit 61 for storing therein. The energy stored in the capacitor storage unit 61 is reused when accelerating the motor 2.
The flywheel storage unit according to the above prior art has the advantage that the amount of energy that can be stored per unit volume of the inertia is large. However, it is inefficient in energy utilization because some of the stored energy is lost due to losses that occur in the inertia rotating motor when storing the energy in the inertia or supplying the energy stored in the inertia. The losses that occur in the motor when storing or supplying the energy, for example, amount to 10 to 20% of the total energy.
On the other hand, the capacitor storage unit according to the above prior art has the advantage that the losses that occur in the large-capacitance capacitor when storing or supplying energy are as small as a few percent or less (in the case of an electrolytic capacitor), i.e., the efficiency of energy utilization is high. Furthermore, since the large-capacitance capacitor can be quickly charged and discharged, the capacitor storage unit has the further advantage of being able to address the situation where it is required to quickly supply drive power to the motor. However, since the amount of energy that can be stored per unit volume is smaller in the case of the capacitor storage unit than in the case of the flywheel storage unit, a sufficient amount of energy may not be able to be stored because of the limited mounting space of the large-capacitance capacitor, and power peaks that occur in the AC power supply may not be able to be suppressed as desired.